1. Field of the Invention
This invention relates to a method for determining the orientation of hydraulically induced fractures in the earth.
2. Description of the Prior Art
Hydraulic fracturing is a production stimulation technique that has been widely used by the oil industry for several years. In a hydraulic fracturing treatment, fluid is injected down a well and into a subterranean formation at rates and pressure sufficient to crack or fracture the subterranean formation. The earth stresses are almost always such that when the rock fails, a vertical fracture similar to that shown in FIG. 5 will be formed. Continued injection of the fracturing fluid increases the fracture length and width. In order to maintain the fracture open, particulate propping agent is injected into the fracture and deposited therein. The conductivity within the propped fracture is substantially higher than the permeability of the formation which changes the flow pattern of produced fluids from a radial flow pattern to a linear flow pattern thereby increasing the productivity of the well.
Since fractures propagate by displacement of the walls of the fracture, the plane of propagation (and, therefore, the orientation of the fracture) is determined by the orientation of the minimum compressive stress in the vicinity of the fracture tip. The fracture normally extends in a direction perpendicular to the least compressive "far field" stress because of the principle of minimum work. The principal rock stress directions are nearly constant over large areas. Thus, the azimuth of the fracture plane will be almost uniform within any reservoir. Knowledge of the azimuthal orientation of the fracture can be of significant value in several oil field operations, as for example, in the selection of injection and producing wells for secondary recovery, since the fluid flow pattern between wells affects efficiency. Also, in the stimulation of wells near fault boundaries or lease lines and in the stimulation of low permeability gas sands by long hydraulic fractures, it would be useful to know the direction of the fracture.
The orientation of a fracture can be determined by several prior art techniques including 1) the use of inflatable impression packers, 2) pressure pulse testing between wells, 3) use of downhole television. All of these techniques, however, either must be performed in an open hole (the inflatable impression packer, downhole television) or lack accuracy (pressure pulse testing). These factors requiring special treating generally militated against the use of such techniques. For example, the pulse testing technique requires the monitoring of pressures in several surrounding wells and, therefore, is time consuming and expensive.
U.S. Pat. No. 3,745,822 describes a technique for determining fracture orientation but relies on an observation well and the effects of thermal energy from an external source such as an input well.
In summary, there is a need in the industry for a simple and reliable technique for determining the orientation of fractures emanating from a cased wellbores.